The invention relates to methods and devices for measuring heat transfer between a specimen or sample of condensed matter and an ambient gas, suitable for use in analysis of that matter; a particularly important application of the method and device consists in the measurement of the light energy absorption spectrum of the specimen.
For several years, there has been an important development in spectroscopy using the conversion into heat of light energy received by the specimen and heat transfer between one surface of the specimen and a gas medium contacting the surface. Such spectrometry techniques can be used in cases where conventional absorption spectroscopy is not suitable, notably in the case of very absorbent, diffusing, solid or semisolid substances.
Prior art techniques of that type consist in directing a monochromatic light flux of varying magnitude onto the specimen located in a gas and detecting the heat transfer between the surface receiving the flux and the ambient gas. Photoacoustic spectroscopy, described in U.S. Pat. No. 3,948,345 (Rosencwaig), uses conversion of heat energy into acoustic energy. A probe (for example a microphone) placed in the vicinity of the specimen supplies electrical signals at the modulation frequency.
Photo-acoustic spectroscopy has drawbacks: the measuring system is very sensitive to mechanical vibration; it requires the use of a measuring cell with thick windows which have an unfavorable effect on the accuracy of the measurements; the size of the samples is obviously limited to that of the cell, generally of small size (typically 3.times.3.times.8 mm.sup.3). Lastly, the probe and the walls of the cell are subject to interference effects.
It is an object of the present invention to remove the above mentioned shortcomings; it is a more particular object to provide a method and a device which have a low sensitivity to external disturbances, while they offer a signal/noise ratio at least signal to the techniques of photo-acoustic spectroscopy and do not impose excessive limitations on the size of the specimens.
According to a feature of the invention there is provided a method for measuring heat transfer between a specimen of a substance in condensed phase and an ambient gas, characterised in that the transfers are detected by measuring the amplitude of the angular displacement of a light pencil which is directed substantially parallel to the surface of the specimen, subject to an energisation whose amplitude varies in time, and which passes through a zone situated in the proximity of said surface.
When the absorption coefficient of the specimen is to be measured, the specimen is energized by a light flux whose magnitude varies with time, advantageously periodically.
In all cases, the light pencil must pass through a zone wherein an appreciable refractive gradient exists, hence very close to the surface. The pencil of light is not necessarily a collimated beam in the form of a right cylinder, but it has sufficiently well-defined boundaries to enable detection of the changes in its angular orientation.
According to another aspect of the invention there is provided a measuring device which comprises a specimen support, means for directing a light pencil parallel to said surface and in the immediate proximity of said specimen and means for detecting the amplitude of the angular displacement of the light pencil.
The energizing light beam is advantageously made monochromatic. By causing the wavelength of the light flux to vary, a variation curve of the deviation as a function of the wavelength, characteristic of the material will be obtained.
However, it is also possible to code the energizing light flux, for example by means of an interferometer with Fourier transform or selective modulation.
This invention will be better understood from the following description of devices which constitute particular embodiments thereof, given by way of nonlimiting examples, and of the method that they utilise.